This invention relates generally to the field of machining three dimensional surfaces on workpieces. More particularly it relates to the cutting of aspheric surfaces on workpieces. The invention finds particular utility in the field of optics in which it is desired to form any of a variety of aspheric surfaces, including toric surfaces, on a workpiece such as a lens blank.
In various fields of activity it is desirable to cut aspheric surfaces on workpieces. One such field of activity in which this is particularly desirable is that of optics, particularly the fields of optometry and ophthalmology, in which corrective lenses are prescribed for individual visual defects. Simple defect such as nearsightedness or farsightedness are corrected by the use of lenses having spherical surfaces. However, more complex defects, such as astigmatism, require a more unusual configuration of lens having at least one aspheric surface.
As described in my previous patent, U.S. Pat. No. 4,680,998, lenses for correcting astigmatism must have a cylindrical, rather than or in addition to spherical, correction. Such a lens providing cylindrical correction will necessarily have a first radius of curvature in one plane or meridian and a second radius of curvature in the second plane or meridian. These two meridians are frequently orthogonal but not necessarily aligned with horizontal and vertical planes intersecting the eye in question. The lens configuration desired is that of a section of the surface of a torus, thus yielding a "toric" lens. This lens provides the necessary cylindrical correction for astigmatism by incorporating two different radii of curvature, one along each of the two orthogonal meridians.
In forming lenses, it is sometimes necessary to provide other aspheric surfaces as well. These may include toric lenses having non-orthogonal axes for the differing radii of curvature, or a bifocal having a sector shape portion of differing correction, or a progressive bifocal with increasing non-spherical refractive power in the lens at greater distances from the center.
With the foregoing explanation as background, the problem becomes how to create the desired contours. In my previous patent, U.S. Pat. No. 4,680,998, I described one form of such an apparatus, in which a conventional lens cutting lathe is provided with an oscillating tool post. In this apparatus a lens blank is mounted to and rotated by the lathe spindle, and a cutting tool is moved in an arc around the end of the workpiece while the cutting tool is oscillated in a sinusoidal motion by a rotary actuator to move the cutting the cutting tool toward and away from the workpiece in synchronization with the rotation of the workpiece. While this apparatus is satisfactory for cutting a predetermined toric surface, the rotary actuator inherently limits the range of types of aspheric surfaces that can be cut.